1. Field of the Invention
The present invention relates to glasses which are capable of being chemically toughened. In particular the present invention relates to glasses which can be chemically toughened and which are primarily, but not essentially, intended for use in aeronautical and automotive vehicles.
In the chemical toughening, of glass, the surface of the glass is compressed by the substitution of alkali ions in the surface layers of the glass by heavier, larger ions. This is customarily effected in an ion-exchange bath containing one or more salts of the heavier ions. By so doing, the breaking strength of the glass is increased, thereby permitting the glass to withstand static stresses, such as those experienced in aircraft cockpits or cabins, and more dynamic stresses, such as those encountered if the aircraft strikes a flock of birds.
2. Description of the Related Art
Chemically strengthenable glasses are well known. Many of these contain significant quantities of lithium. In, for example, U.S. Pat. No. 4,156,755, there is described and claimed such a glass. However, lithium has the disadvantage of increasing the density of the glass and, in many modern applications for chemically toughened glass, this is not acceptable.
German Patent Specification No 19616633C discloses a wide range of glass compositions, some of which overlap the ranges of the present invention. However, the manner in which they are produced is not revealed. These glasses are used for making display panels and security glazing. However, such glasses essentially contain fluorine and cannot, therefore, be made by the float process. Similarly, Russian Patent Specification No 1146288A also discloses glass compositions which overlap some of the ranges of the composition of the present invention. However, these are not made by the float process and, as it is well known by those skilled in the art, there is a very large difference between float (or flat) glass and container glass of the type described in such Patent Specification.
An alternative method of chemically toughening glass if the glass contains sodium ions is to ion-exchange these for potassium ions. Such a method is disclosed in, for example, International (PCT) Patent Application No. WO 94/108910. Such glass has the added advantage that it has a low density of approximately 2.46 in comparison with conventional float glass which has a density of approximately 2.50. Although such patent alleges that no boron need be present in the glass, it is clear that the glass would be more difficult to melt if boron was absent. In fact, this is borne out by the sole example in the patent which discloses a composition containing nearly 3.5% B2O3. Boron oxide lowers the viscosity of the glass. This makes the glass easier to melt and, in theory, easier to refine. Moreover, the combination of, in the context of these glasses, high amounts of both boron and potassium allows the low density to be achieved. However, the use of boron is disadvantageous in that it attacks the silica crowns conventionally used in furnaces.
The use of high quantities of potassium also has drawbacks. In particular, large amounts of potassium cause the production of high viscosity foams early in the melting process. These are very slow to collapse and often lead to silica faults in the finished glass which makes it unacceptable from a commercial viewpoint.
It is desirable if the glasses produced have relatively high strain points so that the ion-exchange can be effected at higher temperatures and the desired level of chemical toughening can be achieved in an economically acceptable time. It is known that the strain point can be raised by increasing the quantities of alumina or zirconia in the glasses. However, these materials are extremely refractory and are difficult to melt in a conventional float furnace within an acceptable time. Alkali metal oxides, such as those of lithium, sodium and potassium help to digest alumina and zirconia but have an adverse effect on the strain point and prevent a high surface compressive stress being achieved during the ion-exchange.
Alkaline earth metal oxides have also been utilised in making glasses which are chemically toughened by ion-exchange. However, these also have drawbacks associated therewith. Zinc oxide is not compatible with the float process, due to the ease with which it is reduced to zinc metal, thereby producing an unacceptable bloom on the glass. Calcium oxide interferes with the sodium/potassium ion-exchange and leads to poor penetration whilst magnesium oxide, particularly in the presence of alumina, normally raises the liquidus temperature of the glass to an unacceptably high level. It will be understood that glasses being manufactured on a float plant should have a positive working range, that is to say, a positive difference between the temperature at which the glass has a viscosity of 10,000 poise and the liquidus, also known as the crystallisation, temperature.
The present invention therefore seeks to provide boron-free glasses having a positive working range, which can be readily melted to float glass standards with respect to the inclusion of bubble and solids and which can be chemically strengthened over a period of less than 100 hours to exhibit a surface stress of at least 400 MPa with a depth of ion penetration greater than 200 microns.
In a subsidiary aspect, the present invention also seeks to provide a glass having a low density. In particular, a low density, in the context of the present invention, is less than 2.48 g/cm3, preferably less than 2.46 g/cm3. This is particularly true if the glass is intended for use in aeronautical applications. Throughout this specification, the amounts of components are given in weight percent unless specifically stated otherwise.
According to the present invention, there is therefore provided a boron-free float glass composition having a positive working range comprising:
with the provisos that the sum of the Al2O3 and MgO exceeds 13%, that the sum of the amounts of Al2O3 plus MgO divided by the amount of K2O exceeds 3 and that the sum of the Na2O plus K2O plus MgO exceeds 22%.
We have surprisingly found that the amount of Al2O3 is critical. If the amount of Al2O3 is less than 5%, insufficient stress can be created when the glass is toughened by ion-exchange but if it is greater than 15%, the glass becomes extremely difficult to melt and causes liquidus problems.
MgO has been found to be a highly desirable component of the glasses of the present invention. It assists in lowering the melting temperature whilst simultaneously not affecting the strain point of the glass. Furthermore, it helps to increase the surface stress of the glass during the ion-exchange process.
Both MgO and Al2O3 help to achieve the high surface compressive stress required if the glass is to be used in aeronautical applications. However, when both are present in comparatively high amounts, as in the glasses of the present invention, they can have an adverse effect on the liquidus temperature of the glass.
K2O poses many problems when melting glass in a float tank. For example, during the melting process, it causes foaming which breaks up into a scum and eventually appears in the finished glass as an inclusion fault. Nevertheless, in the context of the present invention, it is essential to assist in the diffusion of additional potassium ions from the ion-exchange bath so as to achieve sufficiently deep penetration at a reasonable rate. We have surprisingly found that if the amounts of Al2O3, MgO and K2O are as outlined above, the above-mentioned problems do not arise or are at least minimised.
CaO is often used to lower the melting point of glasses. However, its presence in glasses of the present invention lead to low ion penetration during the ion exchange. It is, therefore, not specifically included in the glasses of the present invention but some CaO may be present as an impurity such as, for example, if the glasses of the present invention are being made in a non-dedicated furnace and CaO was present in the composition previously made in that furnace.
In preferred embodiments of the present invention, other constituents may be present in the composition. For example, iron oxide, which gives glasses containing it a green coloration can be used. If used, the iron will be present in both its ferrous and ferric forms. Ferric iron absorbs radiation in the ultraviolet portion of the electromagnetic spectrum whilst ferrous iron absorbs in the infra-red portion. If iron is to be used, the amount thereof and the ratio of ferrous to ferric iron will be selected in dependence, usually, upon the intended use of the glass. Since iron reduces the visible light transmission of the glass, this is clearly undesirable in aeronautical applications and in communication and detection systems which operate using infra-red beams. In such circumstances, therefore, the amount of iron present is maintained low, that is to say, below 0.2%, preferably below 0.05%. Similarly, the amount of ferrous iron present is maintained as low as possible, that is to say, below 20% and ideally below 15% of the total iron present.
On the other hand, if the glass is intended for use in surface vehicles, the absorption of infra-red radiation and ultraviolet radiation are desirable. In such a case, therefore, the amount of iron present may be as much as 2% or higher and the ferrous level may be as high as 40%. To remove the green coloration, other additives such as cobalt, selenium and/or nickel may be included in the composition. If it is desired to improve the ultraviolet absorption of the glass, additives such as cerium or titanium may be included in the composition.
Float glasses are conventionally refined using sulphate, generally identified as SO3, and carbon. However, the usual amounts of these materials added to the composition of the present invention causes excessive foaming. We have found that the glass may be readily refined if the amount of SO3 present in the batch is limited to 0.1% to 0.4%.
The potassium salt used for the ion-exchange may be any suitable salt such as the sulphate, the chloride or mixtures thereof. However, for their general ease of use and for their melting range the nitrates are preferred. The ion-exchange medium may be formed into a paste with an inert medium such as oxides of iron, titanium or silicon as is well known in the art. As is also known per se, the ion-exchange may be accelerated, if desired, by the application of an electric field to the process.
Polished samples of the glasses were strengthened by ion-exchange in a chemical bath containing 99.5% KNO3 and 0.5% silicic acid at temperatures ranging from 400xc2x0 C. to 460xc2x0 C. for periods of from 25 hours to 240 hours. Temperatures of below 400xc2x0 C. may be used but, if a large ion-exchange bath is being employed, there is the possibility of the temperature in localised regions dropping below the melting point of the potassium nitrate. Similarly, temperatures of over 500xc2x0 C. may be employed but, at this temperature level, the potassium nitrate begins to decompose which can have deleterious effects on the quality of the glass and may also release noxious fumes. Following immersion, the glasses were allowed to drain for 30 minutes and were then cooled in air to ambient temperature. Residual salt was washed from the glass and the surface stress and the depth of the compressive layer were measured using a Differential Stress Refractometer. The data was modified to take into account changes in the refractive index of the composition and of the stress optical coefficient. Birefringence was related to applied stress, the comparison being made with a conventional soda-lime-silica glass as made by the float process. The data obtained were combined to predict how long the ion-exchange treatment would need to be continued at a selected temperature to achieve a pre-determined surface stress and compressive depth. Compositions having the desired predicted properties were then prepared as samples for strength testing, were treated for the predicted length of time and were then strength tested to destruction.